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After an introduction, comprising a historical summary on the researches on well fauna, a description of the study area in which 13 water wells have been investigated is given. The authors explain the adopted working method and indicate the physical and chemical characteristics of the waters (temperature, pH, alkalinity, hardness, O2-content, fixed residuum, suspended matter, N2O5, P2O5, NaCl, Ca, Fe). The fauna of the wells of Mehadia (see systematic part) is composed of 34 species: 1 Triclade, 3 Oligochaeta, 2 Gastropods, 5 Cladocera, 1 Ostracod, 3 Copepods, 4 Isopods, 2 Amphipods, 1 Halacarida, 1 Collembola, empty puppies of a Trichoptera, 2 Coleoptera and 8 Diptera (larves and nympha). Among these species 15% can be considered phreatobionts: a blind Triclade (not identified), Candona eremita Vejd., Asellus (Proasellus) danubialis Lt. & M. Codr., Asellus (Proasellus) elegans Lt. & M. Codr., Niphargus jovanovici bajuvaricus Schell. and Niphargopsis trispinosus Dancau & Capuse. The remaining 28 species, counting for 85%, belong to the phreatoxenes. It is worth to mention that Vejdovsky (1882) in wells near Prague, Jaworowski (1895) in wells of Cracovia and of Lwov, Moniez (1888, 1889) in wells in North-East France and Chappuis (1922) in those close to Bale, have found a much smaller proportion of phreatobe forma (e.g. Chappuis 2%).